1. Field of the Invention
This invention relates to apparatus and method for radiation imaging, and particularly to systems and methods for spot or cine photography and television imaging using a common image tube as an image source.
2. Description of the Prior Art
The utility of directing radiation, such as X-rays, through a subject and recording the pattern of this energy emerging from the subject is well known. In the field of medical diagnosis, for example, observation of the pattern of X-rays passing through the subject frequently yields valuable information as to the condition of the subject.
In certain medical applications, radiopaque material is injected into or ingested by the subject and X-rays are then directed through the subject. The accumulation or movement of such material in or about various portions of the body can be observed by noting the pattern of X-rays emerging from the body.
Two basic types of X-ray inspection include television imaging and film camera reproduction of a fluoroscopic image. Television imaging involves production of a continuous image of the region of interest, while film camera inspection is concerned with photographic exposures of an image of the radiation pattern from the subject, where high resolution is essential.
Television imaging is particularly useful in applications in which the areas of differing radiopacity in the subject exhibit motion or are otherwise time varying. Television, for example, can allow the physician to watch the progress of a radiopaque material ingested by the subject through the alimentary canal, or can be used to observe progress of such material injected into blood vessels.
An X-ray fluoroscopy system includes a X-ray source for directing X-rays through the subject, and an image tube located on the side of the subject opposite the X-ray source for converting the pattern of X-rays emerging from the subject to a visible light image. A television camera is positioned to view the image. Other television circuitry produces a display of that image.
A spot camera system includes an X-ray source for emitting X-rays of a relatively high intensity for a short time through the subject, and an image tube positioned as described above in the case of television. A camera having structure for supporting a roll of light sensitive film views the output of the image tube.
Systems have been constructed for combining television and spot camera capabilities, using only a single image tube. The advantages of such combinations of functions are that the examiner can operate continuously in the television mode to observe generally variations in the radiopacity of portions of the subject body, and can make and permanently record spot exposures of high resolution at specific desired times during these radiopacity variations.
Apparatus for performing combined television and spot exposures has typically included an X-ray tube for directing X-rays through the subject, an image tube opposite the source, a spot camera positioned at an angle displaced from the light image produced by the image tube, and a television camera aligned to directly view the light image at the image tube output.
A partially transmissive movable mirror, or beam splitter, is located in front of the image tube output, and acts as a divider of the image tube output image between the spot camera and the television camera. When only television is desired, the mirror is removed from the image tube output light path, allowing all the light from the tube to pass to the television camera. When spot exposures are desired, the mirror is swung into a second position across the image tube output light path, reflecting a portion of the light to the spot camera, from which spot exposures of the image at the at the output can be made.
Spot exposures require relatively high X-ray tube excitation to get a photographable light intensity at the image tube output, e.g., 600 milliamperes (m.a.) for 10-100 milliseconds. Television requires a substantially continuous X-ray output, but with an excitation of only about 7 m.a. or less.
This system has several disadvantages. It is desirable to reduce the time required to actuate the spot camera operation of such systems. It is also desirable to provide a simplified and improved diverter structure and method, for selectively directing the light output of the image tube to the spot camera and the television camera, which is relatively inexpensive, and trouble free.
In the prior systems, the partially transmissive mirror used for selectively dividing the image tube output image is mechanically swung between positions. This mechanical motion requires about one-half second to accomplish. A heavy and complex power source and linkage is required to swing the mirror in this amount of time, because the mirror itself has substantial inertia. Since the mechanical positioning of the mirror at its two positions is crucial in properly deflecting and transmitting the image, considerable adjustment of the mirror is necessary to keep it properly aligned. This tendency to misalignment is aggravated by the necessity for rapid movement of the mirror between positions.
Another factor which limits the speed at which the spot camera's function can be executed is that the X-ray tube requires time to vary its power output between the levels appropriate for television fluoroscopy and spot camera work.
The time lag involved in actuating the spot camera often required the taking of more spot photographs than are necessary. The time lag requires that the examiner anticipate accurately the occurrence of the event whose spot photography is desired. The patient was also often required to cooperate in ingesting radiopaque material in order to synchronize the movement of such material with the efforts of the examiner to photograph it. These techniques often involved a good deal of "cut and try" techniques in order to obtain the desired photographs. Additional required photographs subjected both the patient and the examiner to more radiation than is optimally possible, and also required more time, limiting the number of patients or subjects which can be examined with the system. In previous apparatus with television or optical viewing the exposure produced by the spot camera was undesirably affected because the image was degraded somewhat in the process of being reflected from, or transmitted through, the beam splitting mirror before it reached the spot camera. Any misalignment, mirror distortion or other system imperfections could only increase the image degratation.
In the prior system, the television picture often suffered from "flicker," resulting from undesired variations in light level input to the television camera. Such variations occurred when x-ray levels changed in the course of system operation.
The flickering could occur for several reasons. One cause of flicker was the pulsing of x-ray energy in synchronism with feed of film in the camera. If exposures were being taken at frame rates below about 45 frames a second, flickering of the TV image could not be avoided.
In most systems TV flicker is present whenever film is being exposed even if one does not pulse the x-ray tube. This is true because when energy levels are increased to produce sufficient light to expose the film, the television camera typically receives too much light and this results in a poor, if at all useful, image.
With a perfectly balanced system including an appropriate combination of film, beam splitter and TV camera, together with carefully controlled brightness on the output of the image tube, it is possible to achieve film exposure concurrently with a useful television image. While it is possible, even minute degradation or maladjustment in the system, or change in the film or the desired exposure level of the film, would result in an imbalance and a resultant degrading of the television image. In addition, while it is possible in a perfectly balanced system to achieve flicker-free television, the patient suffers more x-ray dosage than is required and accepted practice is to forego the advantage and pulse the x-ray energy in synchronism with the frame rate of the film camera to minimize dosage to the patient but with a resultant flickering TV image.